S269 Earth and Life
"Atmosphere, Earth and Life"
The Earth contains a large amount of oxygen, some in molecular form, O2, which supports life, and some in combined form in compounds.
There are two reservoirs containing 'free' molecular oxygen available for respiration: the atmosphere and the oceans. The atmosphere contains about 1.2 x 1018kg of molecular oxygen; the oceans contain 7.8 x 1015kg.
The oceans contain about 1.2 x 1021kg of oxygen combined with hydrogen as water, H2O.
The solid Earth consists of about 30% oxygen, combined in the form of minerals. There are about 2 x 1024kg oxygen in combined form in the solid Earth.
Biomass, which can be represented as (CH2O)n and molecular oxygen are formed by photosynthesis from atmospheric carbon dioxide and water using sunlight.
Photosynthesis can be considered as the reverse of aerobic respiration.
During respiration, biomass is oxidized and carbon dioxide and energy are produced.
There are about 2 x 1015kg of living biomass (ie 0.5 x 1015kg of carbon) on land and about 7 x 1012kg in the oceans (ie 2-4 x 1012kg of carbon). Although the ocean biomass is mall relative to that of the land, it is highly influential in the Earth System because of the short time-scales over which changes can take place in the oceans.
The residence time for oxygen in the atmosphere is short relative to the age of the Earth; thus the atmosphere itself is not a major reservoir for oxygen.
If the cyclical processes of photosynthesis and respiration had always been prefectly balanced, oxygen would never have accumulated in the atmosphere. It is only because geological processes cause the system to be one-sided, by burying biomass (ie organic carbon) in sediments, that the amount of oxygen in the atmosphere has been able to build up to its present level of 21%.
Examination of the fossil record enables us to deduce past atmospheric oxygen levels.
Mammals have been abundant for the last 65 million years, so the atmosphere was probably similar to that found today.
Amphibians first came ashore some 380 million years ago - they probably needed a minimum atmospheric oxygen level of between 5 and 10%.
Land plants evolved around 460 million years ago, and led to a rapid 'greening of the land'. Evidence for both charcoal (indicating fire) and mature trees in the early forests shows that atmospheric oxygen levels were probably between 15 and 30% by around 300 million years ago.
Shelly fossils first appeared in the fossil record 540 million years ago, but the first eukaryote fossils are much older, at least 1.7 billion years old. Eukaryotes are thought to need atmospheric oxygen levels of at least 0.2% (0.01 PAL) to metabolize.
Photosynthesizing organisms (e.g. some cyanobacteria) are even more ancient, appearing about 3.6 billion years ago.
The Schidlowski diagram compares the carbon isotope signature (δ13C) in the Ccarb and Corg reservoirs through geological time. Because the two reservoirs show a consistent difference in δ13C over the last 3.5 billion years, it is likely that photosynthesis via the C3 pathway, has been operating throughout that time.
Photosynthesizing prokaryotes are capable of prodigious rates of primary productivity. Thus, even in the most ancient sediments, the amount of buried organic carbon (Corg) is imilar to that in modern sediments, about 0.5-0.6%.
Several strands of evidence in the rock record indicate that a major increase in atmospheric oxygen occurred about 2 billion years ago.
Red beds, which are less than 2 billion years old, are conspicuous components of the rock record. Although the origin of the red colour is complex, it is indicative of an oxidizing environment. They originated in lakes or on river beds, ie they are non-marine.
The origin of banded iron formations (BIFs) is somewhat puzzling, but the huge scale of BIFs required global-scale processes. BIFs were deposited in shallow marine basins. Hydrothermal vents in the deep ocean probably were the source of the reduced iron, which was deposited subsequently in the oxidized form in the iron-rich layers in BIFs. BIFs are predominantly more than 2 billion years old.
The extent of iron leaching in paleosols provides important indirect clues to atmospheric composition. Whether or not iron leaching takes place depends on the oxidation state of the iron in the source rock, and the relative proportions of iron, oxygen and carbon dioxide in the associated soil water. Globally, paleosols suggest a striking change to a more oxygen-rich atmosphere about 2 billion years ago.
Whereas ferric oxides (ie the more oxidized form) are usually highly insoluble, uranium oxides are only soluble in their most oxidized form. A sample of uraninite, the ore of uranium, exposed to an oxidizing atmosphere such as today's would be oxidized and removed in solution. Thus, ore deposits containing grains of detrital uraninite must have been romed in reducing conditions. these deposits are all more than about 2 billion years old.
It is thought that the Earth's mantle was approaching its current oxidation state by 2 billion years ago, and that this allowed the atmospheric oxygen levels to begin to rise, although there is no direct evidence.
Kasting's three-box model provides a useful description of atmospheric evolution. The three boxes are the atmosphere, the surface ocean and the deep ocean. In Stage I, all three boxes are reducing, in Stage II only the deep ocean remains reducing, and in Stage III all three boxes are oxidizing.
The primary atmosphere of a planet is the one that exists immediately after its formation; the secondary atmosphere is then acquired as the result of subsequent changes.
The volatiles that ultimately formed the secondary atmospheres of the terrestrial planets (Mercury, Earth, Venus and Mars) may have arrived in accreted planetary material. Water would have been present chemically bound into the crystal structure of hydrated minerals, or as ice. Carbon and nitrogen would have been present in complex organic compounds, or as methane and ammonia ice. A gaseous molecule can only be retained on a planet if its velocity is less than the planetary escape velocity.
Formation of the Earth's secondary atmosphere was driven by its own internal differentiation and formation of its core. About 80-85% of the Earth's atmosphere was outgassed extremely early in the Earth's history; the remainder has been slowly and steadily released over the last 4.4 billion years. Outgassing has continued at a more modest rate to the present day through volcanoes, which exhale large amounts of water and carbon dioxide.
Molecular oxygen on the early (pre-biotic) Earth, could only be formed by photolytic dissociation of compounds containing combined oxygen (water and carbon dioxide).
Once life evolved at the start of the Archean, photosynthesis became the main mechanism for oxygen generation. There is evidence of large-scale organic carbon burial, but oxygen did not accumulate.
At the present day, photosynthetic oxygen generation is balanced by aerobic respiration, and atmospheric oxygen can only accumulate through burial of organic carbon.
In the early Earth, there was no balance between photosynthesis and respiration, because huge sinks of ferrous (reduced) iron and volcanic gases soaked up photosynthetic oxygen. Methanogenic bacteria also returned large amounts of carbon to the atmosphere in the form of methane, which acts as a sink for oxygen.
There is little evidence for an overall increase in oxygen production with time, so there must have been a decrease in oxygen consumption, perhaps ultimately related to the oxidation state of the mantle.
Over time, the rate of supply of reduced material to the atmosphere waned, while at the same time, geological processes led to increased rates of organic carbon burial, permitting atmospheric oxygen levels to rise at the end of the Archean and into the Proterozoic. Oxidation of ferrous (reduced) iron supplied by deep-ocean hydrothermal vents represented a major sink for oxygen, leading to deposition of BIFs, mostly older than about 2 billion years old.
Carbon isotope data provide the evidence for the fraction of the total carbon which is buried as organic carbon in the sediments, and these data have been used to estimate the mass of organic carbon buried during the Proterozoic. The rate of carbon burial - and changes in atmospheric oxygen levels - show two major increases, one about 2 billion years ago, and the other beginning at about 1 billion years ago. Both have been related to changes in geological activity - increases in rifting and mountain building - that lead to increased burial of sediments.
Although practically all oxygen in the atmosphere is of biogenic origin, increases of atmospheric oxygen levels during the Cryptozoic were dictated by inorganic, geological processes. So evolutionary changes in organisms followed atmospheric change: they did not drive them.
Berner and Canfield modelled levels of atmospheric oxygen over the Phanerozoic by considering the burial rates of pyrite and organic carbon, compared with the weathering rates of pyrite and carbon-bearing sediments. They concluded that the mean atmospheric oxygen level had remained between 15 and 35% over the last 540 million years.
The Berner and Canfield model predicts a large peak in atmospheric oxygen levels between about 350 and 250 million years ago, when the level may have reached a maximum of 35% (1.7 PAL). They ascribed this peak to the evolution of woody plants containing lignin, and also to the existence of a large supercontinent (Pangea) with extensive sedimentary basins, both of which may have promoted enhanced rates of organic carbon burial.
Elevated atmospheric oxygen levels during the late Paleozoic would have led to:
- a higher atmospheric density, as the oxygen would have been an addition ot the atmosphere, and there is no ready mechanism for removing nitrogen;
- an increase in the size (by about 27% for the peak oxygen level) of organisms which respire by diffusion through their body walls via trachea:
- an increase in dissolved oxygen, which could in turn have led to a higher marine biomass.
Elevated atmospheric density and oxygen content may have promoted the evolution of flying insects such as dragonflies, which attained their largest dimensions during the period of the postulated oxygen peak. Elevated oxygen levels would also have encouraged the development of lungs rather than gills and the laying of eggs on land by reptiles, and thus the 'conquest of the land' by animals.
Ozone forms a small but important part of the Earth's atmosphere.
In the stratosphere, ozone is formed through the intereaction of ultraviolet light with oxygen molecules. The presence of ozone causes the temperature to rise within the stratosphere. Both oxygen and ozone absorb ultraviolet radiation effectively, but oxygen absorbs best at very short wavelengths (<0.2 μm) whereas ozone absorbs best at wavelengths between 0.2 and 0.3μm. given its greatere abundance, oxygen is a more effective absorber than ozone at 0.2μm.
Contemporary studies in Canada show that ultraviolet fluxes at the surface are increasing annually, because of destruction of stratospheric ozone by chlorofluorocarbons (CFCs). Observed changes in ultraviolet flux at different wavelengths can be safely attributed to ozone depletion because they correlate well with the marked decrease in ozone's absorption cross-section with increasing wavelength between 0.300 and 0.324 μm.
At wavelengths greater than about 0.3μm, the solar ultraviolet flux is attenuated by the atmosphere. Organisms have eveolved which can utilize these longer wavelengths for photosynthesis: DNA is damaged by shorter wavelength ultraviolet.
Higher levels of ultraviolet flux reaching the Earth's surface as a result of ozone depletion damages modern plant tissues. It follows that low ozone levels may have limited the spread of plants on land and in shallow oceans in the early Earth. Optimal conditions may have been reached at atmospheric oxygen levels as low as 2% (0.1 PAL), or even as low as 0.2% (0.01 PAL).
There is little evidence of ozone levels in the rock record. One interpretation of the banded sediments in the680-million-year-old Elatina formation of Australia, which exhibit a 12-year cyclicity, is that may have been related to intrinsic solar variations. If so, the rhythmically-banded glacial sediments may express temperature variations due to changes in solar flux. Such variations could only have happened at low atmospheric ozone (and oxygen) levels.
Water has played a vital role in keeping the Earth habitable as it helps to remove carbon dioxide from the atmosphere, thus avoiding a runaway greenhouse effect.
Most of the carbon in the Earth's crust is locked away within the rocks and sediments.
We must rely on models to reconstruct how atmospheric CO2 has changed over the Earth's history.
The Earth's earliest secondary atmosphere probably contained high concentrations of CO2 outgassed from its hot interior. Before the emergence of significant land masses, carbon woul dhave been partitioned between the atmosphere, the oceans and the sediments. One estimate puts about 85% of this inorganic carbon in the early sea-floor sediments, giving an upper limit to the level of atmospheric CO2 of over 13 000 PAL, or a pressure of some 7 bar. These estimates are subject to large errors, but it is reasonably certain that the level of atmospheric CO2 was relatively high in the early history of the Earth and that it has decreased steadily over time.
Because there are few data extending as far back as the Hadean, we use the Energy Balance model to calculate the level of atmospheric CO2 which would allow th esurface temperature of the Earth to remain within the range which is consistent with geological evidence. This evidence indicates that the Earth's temperature has remained between 0 and 58 deg C for the last 3.5 billion years.
Results from the Energy Balance model suggest that the level of atmospheric CO2 has declined steeply from between about 7 to 0.1 bar 9about 13 000 to 280 PAL) in the early Hadean to between 10-2 and 10-4 bar (about 30-1 PAL) near the end of the Proterozoic. The main reason for the decline in the level of atmospheric CO2 predicted by the Energy Balance model is the steadily increasing solar radiation. This need not, however, be the sole cause of the decline, since the model is an empirical one. Emergence of land and the evolution of life both accelerated removal of atmospheric CO2.
Paleosol data are broadly consistent with the lowest modelled estimates of atmospheric CO2 from the Energy Balance model.
A likely overestimate of atmospheric CO2 levels by the Energy Balance model could be accounted for if high concentrations of methane augmented the greenhouse effect of CO2 during the Cryptozoic. Also cloudiness, and therefore albedo, may have varied in ways unaccounted for by the model. IT is likely that refining these values used in the Energy Balance model will lead to a lower estimate of the level of atmospheric CO2 levels over the Cryptozoic.
The decline in atmospheric CO2 levels over geological time is caused by the transfer of a significant amount of carbon from the atmosphere to the oceans, and finally to the sediments and rocks.
Modelling the level of atmospheric CO2 over teh Panerozoic is made easier by the abundance of evidence of environmental conditions for that time. This includes a proliferation of shelled organisms in the fossil record, evolution of plants and animals on land, well-preserved sediments, isotope data and buried carbon deposits. These data can be used to recnstruct the long-term sources and sinks of atmospheric CO2.
Results from the GEOCARB model suggest that over the Phanerozoic the atmospheric CO2 level has declined from roughtly 2 x 10-3-7 x 10-3 bar (about 5-10 PAL) to the current level of 3.6 x 10-4 bar. This decline is due to increased weathering rates from an increase in the flux of solar radiation and the evolution and spread of land plants (beginning about 440 million years ago), burial of organic carbon during the Carboniferous, and mountain building (continental uplift) during the Cenozoic. These were offset by increases in CO2 sources through tectonic outgassing during the early to mid-Phanerozoic (the period around 540-400 million years ago) and again around 75-125 million years ago.
Evidence from carbon isotopes preserved in paleosols and sea-floor sediments generally agrees with the modelled results, as does reconstruction of the atmospheric CO2 levels form global mean surface temperature changes over the Phanerozoic.
Scientists have predicted that in the (geologically) near future, levels of atmospheric CO2 could reach up to 0.2% (ie more tha 5 PAL) as a result of human intervention in the carbn cycle over the last several hundred years.
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